1. Field of the Invention
The present invention relates to a method, system and program for selecting a path to a device to use when sending data requests to the device.
2. Description of the Related Art
In certain system configurations, a host may include multiple host bus adapters (HBAs) that provide multiple physical paths to a device, such as a dual ported tape device. In the prior art systems, the host would configure two logical paths to connect to the tape device, where each logical device address provides a separate physical path to the tape device. If a host application, such as a backup application, is writing to a tape drive using one logical device address corresponding to a physical path and experiences a failure, such as a permanent path error (e.g., a failure in the HBA or cable), then the application would cease operating down the failed path. In such case, the administrator may initiate a manual failover to restart the application down the alternate path. However, when restarting down the alternate path, the application would have to start operations from the beginning of the set of operations, losing any I/O requests performed prior to the failure. In the prior art, the application must restart from the beginning, because with sequential devices, such as tape drives, the application is not aware of the location on tape being accessed when failure occurred.
The prior art failover techniques that require the application to start operations from the beginning can be quite costly because the host application may have to resubmit a substantial amount of I/O requests from the time the application started using the path, even if the previously submitted read and write operations successfully completed. For instance, if a backup application encounters a failover while backing up a large storage space to the tape driver, where the application may have been performing the backup for several hours, the backup application would have to restart the backup job from the beginning when switching over to the remaining available path.
For these reasons, there is a need in the art for improved techniques for managing operations with respect to sequential devices, such as tape drives.